Pacemaker Cells
– Electrical power source of the heart
– Dominant cells located in “Sinoatrial” (SA)
node area of heart
– Able to depolarize over and over
– Fires at rate of 60-100 times per minute
• Depends on activity of autonomic nervous system

Depolarization
• Considered an
advancing wave of
+ charges within
heart myocytes. In
turn causes..
• Progressive
contraction of
myocardium

SA Node
• Heart’s dominant pacemaker
• Ability of SA node to generate pacemaking
stimuli is known as automaticity
• Depolarization of atria detected by electrodes
• Noted as “P” wave on EKG
– should not be more than 1 box wide or 1 box tall

Watching for this entity is important b/c it can lead to life threatening
dysrhythmias such as Torsades de Pointes

Shortened QT Etiologies
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•
•
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Digoxin (Digitalis)
Hypercalcemia
Hyperkalemia
Phenothiazines

Plateau & Rapid Phases of
Repolarization

Complete Cardiac Cycle

U Wave
• The U wave is a medical curiosity.
• It is not clear what relationship it has with cardiac activity
but it is thought to represent the repolarization of the HisPurkinje complex.
• Becomes taller in hypokalemia and pts taking Quinidine
• Can flip in CAD.
• Usually follows the direction of the T wave and is best
seen in lead V3.
• Due to the weakness of the signal, the U-wave is often not
seen on the ECG.

U-Wave

J-Point
• The point at which the
QRS meets the ST
segment
• The J point is at the
end of Ventricular
depolarization
• An essential landmark
for measuring QRS
duration

J-Point

Recording the EKG
• Recorded on ruled (graph) paper
• Smallest divisions are 1 millimeter(mm)
long and 1 mm high
• Time
– Horizontal axis represents time
– Each small block is .04 seconds
– Every 5 blocks (between heavy lines) is .2
seconds